Omasum - Anatomy & Physiology

Introduction

The omasum is the third chamber in the ruminant stomach. It lies within the intrathoracic part of the abdomen so cannot be palpated manually. Instead it is examined by ausculation. The omasum has biphasic contractions which squeeze fluid out of the food before allowing the ingesta to continue into the abomasum. Absorption of volatile fatty acids continues in the omasum.

Structure

Omasum Anatomy (Sheep) - Copyright RVC 2008

The omasum is right of the midline. The Rumen and reticulum are located to the left and the liver and body wall to the right. The omasum is covered by lesser omentum and is bilaterally flattened. It is located at ribs 8-11. The lower pole has an extensive connection to the fundic region of the abomasum. An omasal canal is present known as the omasal groove. The floor is smooth except for low ridges and projections around the upper opening. The opening to the reticulum is at the cranial end and the opening to the abomasum is at the caudal end of the omasal canal. The exit to the abomasum is a large oval opening, partly covered by overhanging abomasal folds.

Function

A function of the omasum is absorption. Food enters omasum at second biphasic reticular contraction. The omasum itself has biphasic contractions. The first contraction expels fluid by squeezing the ingesta from the omasal canal between the lamellae. The second contraction expels solids by mass contraction of the omasum. Contractions are slower than the rumenoreticular contractions (see rumination). Food is squeezed between lamellae.

Vasculature

The blood supply to the omasum includes the cranial mesenteric artery, the caudal mesenteric artery and the left gastric and left gastroepiploic arteries.

Innervation

The omasum is innervated by the dorsal vagus nerve (CN X) (most important) and the ventral vagus nerve.

Lymphatics

There are numerous small lymph nodes that are scattered in the omasal curvatures. The lymph drains to larger atrial nodes between the cardia and the omasum, then to the cistera chyli.

Histology

Omasum Histology (Sheep) - Copyright RVC 2008

The omasum has a keratinised stratified squamous epithelium. It is firm in texture and can vary in size. Lamellae thrown into leaves divide the lumen into narrow and uniform recesses. It contains no glands.

The papillae of the omasum are mostly small and lenticular, although some can be large and conical. The circular tunica muscularis extends into papillae of long laminae and the lamina muscualris extends into papillae encircling the tunica muscularis. The omasum has 3 smooth muscle layers in the papillae therefore they are very motile.

Species Differences

Small Ruminant

The small ruminants have a smaller omasum, which is bean shaped.

Bovine

The lower pole of the omasum contacts the abdominal floor below the costal arch.

Links

Click here for information on rumen

Click here for information on reticulum

Click here for information on abomasum

Omasum - Anatomy & Physiology Learning Resources
Flashcards Test your knowledge using flashcard type questions	Omasum flashcards
Videos Selection of relevant videos	Lateral view of the Abdomen of a young Ruminant Sections of the Ruminant Stomach Ovine Omasum and Abomasum Left sided topography of the Ovine Abdomen and Thorax Right sided topography of the Ovine Abdomen Structure of the ruminant forestomachs
PowerPoint Selection of relevant PowerPoint tutorials	Histology of the ruminant gastrointestinal tract